Ethanol (and other biofuels) derived from lignocellulosic materials, such as wood waste, crop stalks and grasses, is a promising renewable energy candidate to replace fossil-based fuels in a variety of applications. Because lignocellulosic materials comprise the majority of terrestrial biomass, producing ethanol from lignocellulosic material has the potential to replace up to 30% of annual petroleum consumption in the United States, while also reducing greenhouse gas emissions. In addition, the use of lignocellulosic biomass in ethanol production does not present the same food production pressures as the use of crops, such as corn.
Converting lignocellulosic biomass to ethanol can involve several steps, including chemical pretreatment of the lignocellulosic material to increase access to hydrolysable cellulose (e.g., by removing lignin), hydrolysis of cellulose to provide fermentable sugars, and fermentation of the hydrolyzed sugars. These steps can be carried out in various ways. Some methods use enzymes such as cellulases to hydrolyze cellulose. However, to achieve complete hydrolysis, a cocktail of different enzymes is typically needed. These enzymes are relatively expensive and generally cannot be reused. In addition, enzymatic hydrolysis of cellulose can be slow, sometimes requiring days or weeks to complete. Further, some methods of converting lignocellulosic biomass to ethanol involve the use of one or more hydrolytic chemical species that can be difficult to separate from desired conversion products and, in some instances, can be toxic to fermentation organisms.